linux-kernel - Re: [PATCH] fs: Fix mod_timer crash when removing USB sticks

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Date:	Thu, 12 Jan 2012 13:38:30 -0800
From:	Greg KH <greg@...ah.com>
To:	Paul Taysom <taysom@...omium.org>
Cc:	Paul Taysom <taysom@...gle.com>, Mandeep Baines <msb@...omium.org>,
	Olof Johansson <olofj@...omium.org>,
	Jens Axboe <axboe@...nel.dk>, Theodore Tso <tytso@...gle.com>,
	Andrew Morton <akpm@...gle.com>, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs: Fix mod_timer crash when removing USB sticks

On Thu, Jan 12, 2012 at 01:15:35PM -0800, Paul Taysom wrote:
> From: Paul Taysom <taysom@...gle.com>
> 
> A USB stick with a ext file system on it, would occasionally crash
> when the stick was pulled.
> 
> The problem was a timer was being set on the Backing Device Interface,
> bdi, after the USB device had been removed and the bdi had been
> unregistered. The bdi would then be later reinitialized by zeroing
> the timer without removing from the timer from the timer queue.
> This would eventually result in a kernel crash (NULL ptr dereference).
> 
> When the bdi is unregistered, the dev field is set to NULL. This
> indication is used by bdi_unregister to only unregister the device
> once.
> 
> Fix: When the backing device is invalidated, the mapping backing_dev_info
> should be redirected to the default_backing_dev_info.
> 
> Created 3 USB sticks with ext2, ext4 and one with both apple and DOS
> file systems on it. Inserted and removed USB sticks many times in random
> order. With out the bug fix, the kernel would soon crash. With the fix,
> it did not. Ran on both stumpy and amd64-generic.
> 
> Change-Id: Icdd06cf3ced555dcd9994cfcc9478a9071a802f1

What is this field for?  It makes no sense for a kernel patch
submission.

> Signed-off-by: Paul Taysom <taysom@...omium.org>
> Downstream-bug-report: http://crosbug.com/24165

Is that a regular field that we now use?

And shouldn't this go to the stable kernel releases as well?

Third time's a charm?

greg k-h
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